In Europe, television broadcast systems utilize a technique for transmitting audio signals, which may be referred to as a near instantaneous companded audio multiplex (NICAM) system. In general, an original sound sample of 14 bits is companded digitally to 10 bits for transmission. A NICAM decoder uses approximately 1 mSec of sound data to lock to an audio frame sequence.
In general, the NICAM system utilizes a high frequency subcarrier (5.85 MHz or 6.522 MHz, depending on the system), which is digitally modulated with a 728 Kbits/sec data stream. The data stream consists of a sequence of 728 bit NICAM packets transmitted continuously (one packet per millisecond). Each NICAM packet includes an 8-bit frame alignment word (FAW) followed by 5 bits of control information, followed by 11 bits of additional data, and a payload packet of 704 data bits. Three of the control bits are mode bits that define how the NICAM decoder should process the data bits of the payload packet. Another control bit identifies to the decoder if the frequency modulated (FM) television audio may be utilized as a fallback if the NICAM signal fails. The first control bit toggles between a logic high level and a logic low level to allow identification of odd and even packets. The first control bit toggles high for eight packets and then low for eight packets.
Since the FAW is just a sequence of bits and since nothing prevents the bit pattern of a FAW from appearing within the payload data, the detection process typically requires identifying a FAW, determining if the identified FAW is a real FAW (as opposed to a data pattern resembling a FAW within the payload), and then locking to the frame sequence using the FAW. Typically, a decoder detects a possible FAW and the first control bit after the FAW (referred to as the C0 bit), and then monitors each subsequent frame to determine whether the possible FAW repeats and to determine when the first control bit changes. Within 8 frames, the first control bit should transition from logic high to logic low or from logic low to logic high.
If a FAW is correctly identified, when the first control bit changes, the detector begins counting the frames to determine a lock. The first control bit should change every eight frames, so the FAW should remain unchanged for the eight frames, or the detector may restart the process. However, it may be difficult, with bit errors, to detect the FAW and the first control bit for up to 16 consecutive frames. Since NICAM transmissions may experience noise, the detection and locking process may require multiple iterations to lock to the frame sequence, even if the noise creates only a minor error in the FAW, such as only a single bit error in each FAW. When a system generating digital sound from the NICAM signal loses synchronization due to noise, a lengthy re-acquisition of synchronization will delay the return to digital sound.
Therefore, there is an on-going need for a detector to efficiently detect a frame sequence and to synchronize to the frame sequence in the presence of a noisy NICAM signal. Furthermore, there is also an on-going need for a detector to maintain synchronization in the presence of a noisy NICAM signal.